Sterilization of goods may be carried out in batch processes, wherein a number of goods, called the load, is loaded into a chamber of appropriate size and subjected, e.g., to a temperature cycle ensuring a specified degree of sterility, that is lack of viable microorganisms. Chemicals may also be employed, e.g. the use of peroxides and ethylene oxide are well known, but the majority of such sterilization processes work by means of heating, often with steam. With steam in this context, and for the purpose of this text, is meant water in gaseous form or condensing at its dew point. Vapor for the purpose of this text means liquid dispersed in a gas.
Sterilization processes often include rinsing, cooling or drying stages, typically using clean air.
The apparatus employed has developed from the traditional simple autoclave to advanced, GMP-validated sterilizers programmable for a multitude of different tasks.
The specific process used for a particular sterilization task is dependent on the characteristics of the load. Solid materials, which may be porous or have a complicated structure may be subjected to a vacuum, which assists in replacing the initial surrounding atmosphere with heat transfer medium. Other loads, like liquids packed in vented containers or bottles, vials, ampoules, pouches, blister pads and the like, may not tolerate vacuum and must be heat treated at pressures from atmospheric upwards.
As the heat stability of many types of loads is limited, and also due to economical reasons, the load must often be cooled when the specified amount of heat has been delivered. Thus, the processes routinely include stages of active cooling. If the load must be dry when leaving the sterilization process, this also requires tailoring of the procedures.
It is a well-known physical fact that convection is a more efficient mechanism in heat transfer than conduction. Also, the conditions within a sterilization chamber must be as uniform as possible to ensure sterilization efficiency regardless of the location and shape of a particular piece of load. Thus, the distribution of the medium surrounding the load during all stages of the process is important, and the medium should be kept in motion. Also, it is well known that removing moisture from the treated goods is more efficient if the moisture-transferring media can be kept efficiently in circulation. According to the prior art, this has been accomplished using fans. A fan in a sterilization chamber mostly requires a shaft penetrating the chamber wall, which leads to expensive technical solutions. Lubricants other than pure water are usually not allowed in sterilization chambers, and this fact puts requirements on the bearings. Magnet-coupled drives eliminate the shaft but not the bearing issue, and they are expensive.
In addition to proper medium distribution, a fan provides the dynamics required for penetration of gaseous medium into the crevices of the load.
In U.S. Pat. No. 2,713,702, a low-temperature autoclave is disclosed having a conventional steam jet aspirator discharging into the autoclave chamber. The system includes a return line from the chamber to the aspirator and is designed for providing intermittent injection of steam into the chamber to maintain a uniform, controlled temperature below 100° C. without creating a vacuum.